Generally, the data conveyed in such systems is binary data in the form of pulses clocked at a determined clock frequency. The amplitude levels of the pulses are representative of the binary data. Initially, the pulses are in electrical form, and then they are converted into an optical signal obtained by modulating the power (or the amplitude) of an optical carrier wave.
Prior to feeding the optical signal into the system, it is desirable for the optical pulses making up the signal to be properly adjusted with respect both to phase and to amplitude. However, even if the signal to be transmitted is well adjusted on being generated, in general it is transformed into an optical signal after it has passed through various electrical and/or optical apparatus, the effect of which is to impart fluctuations in amplitude and in phase (jitter). The effect of such fluctuations is to increase receiver error rate, thereby putting a limit on transmission rate and/or distance.